1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and a system of fabricating a semiconductor device, and more particularly, it relates to a method of fabricating a semiconductor device including a step of heating a semiconductor layer with a laser beam and a system of fabricating a semiconductor device.
2. Description of the Background Art
A thin-film transistor (hereinafter referred to as a polycrystalline silicon TFT) employing a polycrystalline silicon film as an active layer has recently been employed as a pixel driving transistor for a liquid crystal display. In such a liquid crystal display, the performance of the polycrystalline silicon TFT must be improved in order to reduce the cost, improve the performance and render the display lightweight and compact. In order to improve the performance of the polycrystalline silicon TFT, a polycrystalline silicon film formed on a substrate must be converted to a single-crystalline state as much as possible.
A technique of heating a semiconductor layer with a laser beam thereby crystallizing the semiconductor layer or activating an impurity contained therein is known in general. A technique of heating a semiconductor layer by excimer laser annealing (ELA) thereby obtaining a larger crystal grain size has recently been developed. This ELA is employed for performing crystallization in a short time by pulse oscillation of several 10 nsec in order to avoid a thermal effect on a substrate as a mainstream technique of crystallization in a low-temperature process employing a low-priced glass substrate. Further, the ELA employing a laser beam having a short wavelength of about 300 nm attains high absorptivity in amorphous silicon or polycrystalline silicon. Thus, a silicon film can be heated to a high temperature in a short time.
In the aforementioned conventional excimer laser annealing (ELA), however, the absorptivity is remarkably influenced by the thickness or the quality of the heated semiconductor layer and the beam intensity is dispersed due to instability of pulse oscillation, and hence it is difficult to homogeneously heat the semiconductor layer. Therefore, device characteristics are inconveniently dispersed to reduce the yield. Further, the conventional ELA inconveniently results in a high device cost or a high operation cost. In addition, it is difficult to perform high-speed scanning with the laser beam due to the pulse oscillation. Thus, the throughput (productivity) is disadvantageously reduced.
The inventor has proposed a technique of crystallizing an amorphous silicon film with a continuous-wave YAG laser beam in Japanese Patent Laying-Open No. 2001-291666 or 2002-50576. According to this proposed method, it is possible to form gigantic crystal grains or a gigantic single crystal at a low cost and high throughput without reducing the yield.
In the aforementioned conventional method employing a YAG laser beam, however, a laser oscillator and an irradiation optical system are connected with each other through an optical system including a mirror, a lens or the like, leading to difficulty in miniaturizing the overall optical system including the irradiation optical system. Further, all of the laser oscillator, the irradiation optical system and the optical system for connecting the laser oscillator and the irradiation optical system with each other must be installed in a clean room (dust-free room), and hence the size of the clean room is inconveniently increased.
In this regard, a technique of connecting a laser oscillator and an irradiation optical system with each other through an optical fiber bundle having a plurality of core parts is proposed in general. This technique is disclosed in Japanese Patent Laying-Open No. 6-345415 (1994) or 2001-156017, for example. When the laser oscillator and the irradiation optical system are connected with each other through the optical fiber bundle, it is possible to miniaturize the overall optical system including the irradiation optical system. When the optical fiber bundle is employed, further, only the irradiation optical system may be installed in a clean room, which in turn can be prevented from size increase.
In the aforementioned method employing the optical fiber bundle formed by bundling a plurality of optical fiber members, however, it is so difficult to introduce a laser beam condensed by a lens only into the plurality of core parts of the optical fiber bundle that the laser beam is also incident upon cladding parts located around the core parts. In this case, the temperatures of the cladding parts are inconveniently increased to cause seizure if a high-density beam is incident upon the same, and hence it is difficult to remarkably increase the density of the beam incident upon the optical fiber bundle. In the conventional method employing the optical fiber bundle, therefore, it is difficult to introduce a laser beam having high optical density necessary for crystallizing a semiconductor layer or activating an impurity contained therein into the optical fiber bundle. Consequently, it is difficult to crystallize the semiconductor layer or activate the impurity in the conventional method employing the optical fiber bundle.
The aforementioned Japanese Patent Laying-Open No. 6-345415 describes a method of reducing the diameter of the optical fiber bundle on an outlet side thereby improving the optical density of the laser beam. According to this method, however, the optical density cannot be so much improved and hence it is difficult to obtain a high-density laser beam necessary for crystallizing the semiconductor layer or activating the impurity.
The density of the laser beam outgoing from the outlet of the optical fiber bundle may be remarkably increased (the laser beam may be contractedly projected) through a lens group provided on the outlet, thereby improving the optical density of the laser beam. In this case, however, the size of the lens group provided on the outlet of the optical fiber bundle is disadvantageously increased.
In the conventional method employing the optical fiber bundle, as hereinabove described, it is difficult to obtain a high-density laser beam necessary for crystallizing the semiconductor layer or activating the impurity while miniaturizing the lens group provided on the outlet of the optical fiber bundle.